Method and apparatus for resolving call collisions in a digital conventional direct mode

ABSTRACT

A process for resolving call collisions in a digital conventional direct mode includes monitoring a direct mode communication channel for transmissions from other direct mode radios in the plurality of direct mode radios. In response to detecting a new call request: identifying a last radio to transmit on the direct mode channel, transmitting a new call request for receipt by the last direct mode radio to transmit, monitoring the direct mode channel for a response from the last radio to transmit, and if a call grant granting the new call request is received from the last radio to transmit, initiating the new direct mode call on the direct mode communication channel. If the call grant is not received, at least temporarily refraining from initiating the new direct mode call.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to wireless communications andmore particularly to direct mode call collision avoidance in a digitalconventional direct mode communications system.

BACKGROUND

The European Telecommunications Standard Institute-Digital Mobile Radio(ETSI-DMR) is a direct digital replacement for analog Private MobileRadio (PMR). DMR is a scalable system that can be used in unlicensedmode (in a 446.1 to 446.2 MHz band), and in licensed mode, subject tonational frequency planning ETSI standards or specifications referred toherein may be obtained by contacting ETSI at ETSI Secretariat, 650,route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE.

DMR provides improved range, higher data rates, more efficient use ofspectrum, and improved battery life over prior direct mode protocols.Features supported include fast call set-up, calls to groups andindividuals, and short data and packet data calls. Supportedcommunications modes include individual calls, group calls, andbroadcast calls provided via a direct communication mode among theradios operating within the network. Other direct mode functions such asemergency calls, priority calls, full duplex communications, short datamessages and Internet Protocol (IP)-packet data transmissions are alsosupported.

Direct mode, more generally, is a mode of operation where radios maycommunicate within a network without the assistance of one or moreinfrastructure equipment (e.g., base stations or repeaters). A radio, asused herein, can be mobile and/or fixed end equipment that is used toobtain direct mode communications services. Direct mode operation iscontrasted with a conventional repeater mode, which is a mode ofoperation where radios communicate with one another throughinfrastructure equipment such as a repeater or base station. Directmode, therefore, can provide a more efficient and less costlycommunication system operation than repeater mode operation.

The ETSI-DMR standard provides for 12.5 Kilohertz (KHz) operation indirect mode (systems that exclusively communicate without a repeater).The 12.5 KHz operation refers to 12.5 KHz spectral efficiency in whichthere are two communication paths per 12.5 KHz of radio frequency (RF)spectrum. The 12.5 direct mode utilizes 27.5 millisecond (msec) pulsed(every 60 msec) radio transmissions on each of the logical channels. Inthe 12.5 direct mode of operation, radios transmit asynchronously andradios within range of the transmission synchronize themselves to thattransmission for the purposes of receiving the transmission, but anytransmissions in response to the first transmission are transmittedasynchronously.

Other direct mode protocols, perhaps consistent with the Project 25(P25) standard defined by the Association of Public SafetyCommunications Officials International (APCO) and standardized under theTelecommunications Industry Association (TIA), or with the terrestrialtrunked radio (TETRA) standard also defined by the ETSI, may operate ina similar manner and may be used in addition to or in place of the DMRprotocol. Communications in accordance with any one or more of thesedirect mode communication standards, or other standards, may take placeover physical channels in accordance with one or more of a TDMA (timedivision multiple access), FDMA (frequency divisional multiple access),or CDMA (code division multiple access) protocol.

In prior analog direct mode communications systems, two or moresimultaneous direct mode transmissions can be mixed at the RF level andthe recipient thus able to hear the mixture of all the conversations.However, in a digital conventional direct mode consistent with any oneof the foregoing mentioned standards, two or more sources transmittingsimultaneously would corrupt both of the digital transmissions and, as aresult, the recipients would not hear any conversation. Even worse, thesimultaneously transmitting digital radio users would not be aware ofthe corrupted transmission and may (incorrectly) assume that thetransmissions reached their respective targeted group of recipientdigital direct mode radios. This issue is further exacerbated in digitaldirect mode group calls, where the number of digital direct mode radiosin the group (e.g., more than five) increases the odds that simultaneoustransmissions would conflict.

Accordingly, there is a need to provide an improved call collisionarbitration process and apparatus in digital conventional direct modecommunication systems in order to improve the robustness of thecommunications systems and ensure that digital transmissions are notcorrupted by multiple simultaneous direct mode digital transmissions.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separateviews, together with the detailed description below, are incorporated inand form part of the specification, and serve to further illustrateembodiments of concepts that include the claimed invention, and explainvarious principles and advantages of those embodiments.

FIG. 1 is a block diagram of an illustrative digital conventional directmode wireless communications system operating in accordance with anembodiment.

FIG. 2 is a timing diagram illustrating one example of a calltransmission in a digital conventional direct mode wirelesscommunication system in accordance with an embodiment.

FIG. 3 is a block diagram of a direct mode wireless radio operable inthe digital conventional direct mode wireless communications system ofFIG. 1 in accordance with some embodiments.

FIG. 4 is a flowchart illustrating a process, executable at aninitiating direct mode radio for resolving call collisions in a digitalconventional direct mode wireless communication system in accordancewith some embodiments.

FIG. 5 is a flowchart illustrating a process, executable at anarbitrating/last transmitting direct mode radio for resolving callcollisions in a digital conventional direct mode wireless communicationsystem in accordance with some embodiments.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

The apparatus and method components have been represented whereappropriate by conventional symbols in the drawings, showing only thosespecific details that are pertinent to understanding the embodiments ofthe present invention so as not to obscure the disclosure with detailsthat will be readily apparent to those of ordinary skill in the arthaving the benefit of the description herein.

DETAILED DESCRIPTION

In light of the foregoing, it would be advantageous to provide for amethod and device that monitors direct mode communications, detects newcall requests, identifies a last radio to transmit, transmits a new callrequest to the last radio to transmit, monitors for a response from thelast one of the other direct mode radios to transmit on the direct modecommunication channel, and acts accordingly based on whether or not anacknowledgement of the call request is received from the last one of thedirect mode radios to transmit. Similarly, it would be advantageous toprovide for a method and device for a last-to-transmit radio toarbitrate and grant or not grant subsequent new call requests in adirect mode communication channel.

In one embodiment, a process for resolving call collisions in a digitalconventional direct mode includes monitoring a direct mode communicationchannel for transmissions from other direct mode radios in the pluralityof direct mode radios. In response to detecting a new call request:identifying a last radio to transmit on the direct mode channel,transmitting a new call request for receipt by the last direct moderadio to transmit, monitoring the direct mode channel for a responsefrom the last radio to transmit, and if a call grant granting the newcall request is received from the last radio to transmit, initiating thenew direct mode call on the direct mode communication channel. If thecall grant is not received, at least temporarily refraining frominitiating the new direct mode call.

In another embodiment, a process for resolving call collisions in adigital conventional direct mode includes transmitting, on a direct modecommunication channel, one or more transmissions during a first directmode call to the other direct mode radios in the plurality of directmode radios, the transmissions including an identifier identifying thefirst direct mode radio, and after making the last transmission of thefirst direct mode call, transitioning to an arbitrating mode andmonitoring the direct mode communication channel for any new direct modecall requests from other direct mode radios in the plurality of directmode radios. While monitoring the direct mode channel, receiving, from asecond direct mode radio out of the plurality of direct mode radios, afirst new direct mode call request, and continuing to monitor the directmode communication channel for any additional new direct mode callrequests from other direct mode radios in the plurality of direct moderadios. Responsive to determining that (i) no other new direct mode callrequests have been received or (ii) other new direct mode call requestshave been received but the first new direct mode call request isentitled to priority over the other new call requests: broadcasting acall grant granting the first new direct mode call request of the seconddirect mode radio, the call grant including an identifier identifyingthe second direct mode radio, and receiving and playing back contentreceived from the second direct mode radio broadcast over the directmode communication channel during the first new direct mode call grantedby the acknowledgment.

Each of the above-mentioned embodiments will be discussed in more detailbelow, starting with example network and device architectures of thesystem in which the embodiments may be practiced, followed by adiscussion of resolving call collisions in a digital conventional directmode communication system from the point of an initiating radio and thenfrom the point of view of an arbitrating/last transmitting radio.Further advantages and features consistent with this disclosure will beset forth in the following detailed description, with reference to thefigures.

I. Network and Device Architectures

Referring now to FIG. 1, an example of a digital conventional directmode wireless communications system 100 comprising a plurality of radios105 operating in accordance with an embodiment is illustrated. Radios105-1 through 105-5 communicate with each other on direct mode radiofrequencies without communicating through any infrastructure including,for example, a repeater or base station. It will be appreciated by thoseof ordinary skill in the art that in some embodiments the frequencycould also have repeaters on it, but radios 105 may not use thoserepeaters (e.g., the repeaters could belong to a different system, orthe radios are operating in a talk-around mode).

A radio, as used herein, can be mobile and/or fixed end equipment thatis used to obtain direct mode wireless communications services. Forexample, a radio can be a mobile radio (i.e. a portable radio, a mobilestation, a subscriber unit, a mobile subscriber), or can be a fixedstation (i.e. a fixed control station, a base station, and anysupporting equipment such as wireline consoles and packet dataswitches). Each radio is capable of communicating directly with one ormore other radios using techniques as further described herein, such asTDMA, in which specified time segments are divided into assigned timeslots for individual communications and each radio frequency (RF) in thesystem carries time slots whereby each time slot is known as a“channel.” In the case of FDMA communications, channels may be separatedby frequency, and in CDMA communications, channels may be separated byspreading code.

For ease of describing the embodiments hereinafter, the digitalconventional direct mode wireless communications system 100 is presumedto be a two time slot TDMA communications system in accordance with theETSI-DMR standard. Thus, in the embodiments described below, since thereare two time slots, there are two channels available on each radiofrequency for carrying the traffic of the system. A time slot is anelementary timing of the physical channel. For example, in oneembodiment consistent with the ETSI-DMR standard for repeater-basedcommunications, a time slot has a length of thirty milliseconds (30 ms)and is numbered “1” or “2”. In another example, and consistent with theETSI-DMR direct mode, only one of the two time slots is used for directmode communications, leaving one 27.5 ms timeslot numbered “1” (aftereliminating unnecessary 1.25 ms guard intervals). It is important tonote, however, that the TDMA communication system may have other slotlengths and slotting ratios, as well.

As discussed above, and illustrated further in the transmission diagram200 of FIG. 2, within the present ETSI-DMR direct mode systems, in the12.5 KHz direct mode of operation, radios transmit asynchronously(because there is no common time slot reference available) and radioswithin range of the transmission synchronize themselves to thattransmission for the purposes of receiving the transmission, but anytransmissions in response to the first transmission are transmittedasynchronously. At present, and in one mode, there is an unused 32.5 msportion 205 of the channel according to the standard (used in therepeater mode for a 2^(nd) timeslot/channel). In another mode, anadditional 2^(nd) timeslot/direct mode channel is provided in the unused32.5 ms portion 205, and the radios transmit synchronously.

Radios 105-1-105-5 may directly communicate over a single shared RFfrequency in accordance with the ETSI-DMR standard. For example, radio105-1 in digital conventional direct mode communication system 100 mayinitiate a new direct mode call 114 to radios 105-2-105-5 on a timeslot1 202, as illustrated in the timing diagram 200 of FIG. 2. Timeslot 1202 includes a 27.5 ms payload period 214 that includes a sync slot 216.At substantially the same time that radio 105-1 initiates its new call,radio 105-5 in digital conventional direct mode communication system 100may, having no knowledge of the direct mode call 114, similarly initiatea new direct mode call 124 to radios 105-1-105-4 on the same direct modechannel (e.g., during the same timeslot 1 202 of FIG. 2). Assuming thereis no arbitration on the channel, radio 105-1's and radio 105-5'stransmissions during a same time period would likely corrupt each other,and as a result, radios 105-2-105-4 would not receive either of thedigital calls being transmitted by radios 105-1 and 105-5. FIGS. 4 and5, below, illustrate (with respect to FIGS. 1 and 2) a method forresolving this type of call collision in digital conventional directmode communications systems.

FIG. 3 is an example functional block diagram of a direct mode radio,such as radio 105-1 operating within the system 100 of FIG. 1 inaccordance with some embodiments. Other direct mode radios, such asradios 105-2-105-5, may contain same or similar structures. As shown inFIG. 3, radio 105-1 includes a communications unit 302 coupled to acommon data and address bus 317 of a processing unit 303. The radio105-1 may also include an input unit (e.g., keypad, pointing device,etc.) 306, an output transducer unit (e.g., speaker) 320, an inputtransducer unit (e.g., a microphone) 321, and a display screen 305, eachcoupled to be in communication with the processing unit 303.

The processing unit 303 may include an encoder/decoder 311 with anassociated code Read Only Memory (ROM) 312 for storing data for encodingand decoding voice, data, control, and/or other signals that may betransmitted or received between other radios within direct modecommunication range of radio 105-1. The processing unit 303 may furtherinclude a microprocessor 313 coupled, by the common data and address bus317, to the encoder/decoder 311, a character ROM 314, a Random AccessMemory (RAM) 304, and a static memory 316. The processing unit 303 mayalso include a digital signal processor (DSP) 319, coupled to thespeaker 320, the microphone 321, and the common data and address bus317, for operating on audio signals received from one or more of thecommunications unit 302, the static memory 316, and the microphone 321.

The communications unit 302 may include an RF interface 309 configurableto communicate directly with other direct mode radios, such as radios105-2 to 105-5. The communications unit 302 may include one or morewireless transceivers 308, such as a DMR transceiver, an APCO P25transceiver, a TETRA transceiver, a Bluetooth transceiver, a Wi-Fitransceiver perhaps operating in accordance with an IEEE 802.11 standard(e.g., 802.11a, 802.11b, 802.11g, 802.11n), a WiMAX transceiver perhapsoperating in accordance with an IEEE 802.16 standard, and/or othersimilar type of wireless transceiver configurable to communicate via awireless network. The transceiver 308 is also coupled to a combinedmodulator/demodulator 310 that is coupled to the encoder/decoder 311.

The microprocessor 313 has ports for coupling to the input unit 306 andto the display screen 305. The character ROM 314 stores code fordecoding and/or encoding data such as control messages and/or data orvoice messages that may be transmitted or received by the radio 105-1.Static memory 316 may store operating code for the microprocessor 313that, when executed, monitors direct mode communications, detects newcall requests, identifies a last radio to transmit, transmits a new callrequest for receipt by the last radio to transmit, monitors for aresponse from the last one of the other direct mode radios to transmiton the direct mode communication channel, and acts accordingly based onwhether or not an acknowledgement of the call request is received fromthe last one of the direct mode radios to transmit, in accordance withone or more of FIGS. 4-5 and corresponding text. Static memory 316 mayadditionally or alternatively store operating code for themicroprocessor 313 that, when executed, allows a last-to-transmit radioto arbitrate and grant or not grant subsequent new call requests in adirect mode communication channel in accordance with one or more ofFIGS. 4-5. Static memory 316 may comprise, for example, a hard-diskdrive (HDD), an optical disk drives such as a compact disk (CD) drive ordigital versatile disk (DVD) drive, a solid state drive (SSD), a flashmemory drive, or a tape drive, to name a few.

II. The Process of Resolving Call Collisions in a Digital ConventionalDirect Mode

FIGS. 4-5 set forth examples of a method for resolving call collisionsin a digital conventional direct mode, from the point of view of, andexecutable at, a new call initiating radio device (FIG. 4) and then fromthe point of view of, and executable at, an arbitrating/lasttransmitting radio device (FIG. 5). In the following process flowdiagrams, examples will be described with respect to FIGS. 1 and 2, inwhich it is assumed that radios 105-1 to 105-5 are all on a same system(e.g., use the same color code or other system identifier to distinguishtheir calls from other calls that may also occur on a same direct modefrequency), 105-2 was the last radio transmit on the system, and thatradio 105-1 transmits a new call request first.

FIG. 4 sets forth an example process flow 400 in which a new callinitiating radio device, perhaps radio 105-1 of FIG. 1, helps to resolvecall collisions in a digital conventional direct mode. At step 402, aninitiating direct mode radio monitors a direct mode channel (e.g.,frequency, time slot, and/or code) with which it is associated fortransmissions from other direct mode radios that transmit with a samesystem (or group) identifier with which the initiating direct mode radiois associated. For example, radio 105-1 monitors a direct mode TDMAchannel for transmission from other radios (e.g., 105-2 to 105-5) thatidentify with the same system identifier or group identifier. In oneembodiment, the system identifier may be a color code, and the radio105-1 may monitor a direct mode TDMA channel for transmission from otherradios with a same color code as it is configured with (e.g., 0001,0101, or 1111). In another embodiment, the system identifier may be agroup identifier such as a talkgroup identifier.

If a transmission is detected at step 402 from a radio on a system orgroup as the monitoring radio, the initiating radio decodes thetransmission and locates a source ID for the transmission (identifyingthe source radio), and records the source ID in local storage as alast-transmitting radio. The source ID could be a hardware identifierpre-provisioned into the radio (perhaps in accordance with the DMRstandard), a MAC address, an IP address, or some other identifiercapable of uniquely identifying the radio doing the transmitting. Thetransmission containing the source ID could be a link control (LC)burst, an embedded LC burst, a voice header, a preamble controlsignaling block (CSBK), or any other signaling, voice, or data burstthat includes a source ID identifying the transmitting radio. In someembodiments, call request and call grant bursts are not considered inidentifying the last radio to transmit.

During the monitoring of step 402, transmissions received that include asystem identifier or talkgroup identifier that is not associated with asame system identifier talkgroup with which the initiating radio isassociated may simply be discarded, without recording or otherwisestoring the identity of the radio doing the transmitting.

At step 404, the initiating radio detects a request to initiate a newdirect mode call on the direct mode channel. For example, direct moderadio 105-1 of FIGS. 1 and 3 may detect a request to transmit a newdirect mode call via a detected activation of a PTT input 306 indicatinga user's desire to transmit a new direct mode call (perhaps as afunction of an indicated group or channel selected via a separatechannel knob input 306). Other types of inputs, includingvoice-activated or touch-activated, could additionally or alternativelycause the initiating radio to detect a request to initiate the newdirect mode call. Also at step 404, the direct mode radio sets aninternal retry counter to 0.

At step 406, and responsive to detecting the request to initiate the newdirect mode call, the initiating radio identifies the last direct moderadio to transmit (e.g., the most recent or immediately previous radioto transmit) on the direct mode channel. In one embodiment, this may bethe last direct mode radio to transmit with a same system identifier orgroup identifier with which the initiating radio is also associated, andmay be a source ID retrieved from storage after the monitoring at step402. In some embodiments in which there is no known last transmittingradio, or the previously identified and/or stored last transmittingradio actively has indicated that it is powering down or otherwiseleaving the area and would no longer be available to arbitrate directmode calls, the initiating radio may identify a transmitting radio priorto the last transmitting radio, may identify itself as the lasttransmitting radio, or may assign a random or pseudo-random identifieras the last transmitting radio source ID. Other possibilities exist aswell.

At step 408, and assuming the direct mode channel is currently idle, theinitiating radio transmits a new call request on the direct mode channelfor receipt by the last known transmitting radio that was identified atstep 406. The request may be, for example, a voice header or a preamblecontrol signaling block (CSBK), or any other signaling, voice, or databurst acting as a new call request and populated with the source ID ofthe identified last direct mode radio from step 406 as the target devicefor the new call request. The request may be transmitted directly to thelast direct mode radio (e.g., identify the last direct mode radio as atarget of the request), or may be transmitted or broadcast in some othermanner for receipt by the last direct mode radio (e.g., identify someother radio such as the desired target radio for the new call, in whichcase the last radio to transmit may still be configured to intercept andrespond to such a formatted request). Other examples are possible aswell.

At step 410, the initiating radio switches from transmit mode to receivemode and monitors the direct mode channel for a response from theidentified last direct mode radio (e.g., the “arbitrating radio” for thedirect mode call). At step 412, the initiating direct mode radiodetermines if the arbitrating radio granted its call request or not. Forexample, it may monitor the direct mode channel for a call grant fromthe identified last direct mode radio that includes the initiatingradio's source ID. In one embodiment, the call grant may be a repeatedversion of the call request, with flipped source and destinationidentifiers, and could be transmitted in the form of a call header orCSBK.

Assuming a call grant is received at step 412 granting the initiatingdirect mode radio's new call request, processing proceeds to step 414,where the initiating direct mode radio transmits the new call on thedirect mode channel. The new call may be a private call (radio toradio), a group call (radio to group via multicast or broadcast), or abroadcast call (radio to all).

On the other hand, if the initiating direct mode radio does not receivethe call grant granting the requested new call at step 412 within somethreshold period of time (such as between 60 and 180 ms, and in oneembodiment, 120 ms), processing proceeds to step 416, where theinitiating direct mode radio refrains from initiating the new call forat least a temporary period of time (e.g., until some furtherdetermination is made), up to a configurable maximum period of time suchas 180 ms. Further at step 416, the initiating direct mode radio maydetermine at step 416 whether a call grant is received that identifiessome other radio than the initiating direct mode radio, but e.g., withthe same system or group identifier as the initiating radio. Forexample, the radio 105-1 may monitor the direct mode channel and see acall grant (perhaps with a same color code of “0001” with which may beassociated) from arbitrating/last known transmitting radio 105-2 thatidentifies radio 105-5 as the target of the call grant. If it isdetermined that a new call grant was transmitted granting a new call tosome radio other than the initiating radio, processing proceeds to step418, where the initiating direct mode radio determines whether the callgranted to the other radio is of interest to the initiating direct moderadio (e.g., the grant identifies a same color code and/or group withwhich the initiating direct mode is associated or subscribed).

If the call is of interest, the initiating direct mode radio joins thenew call on the direct mode channel at step 420, including receiving,decoding, and playing back content (e.g., voice, audio, and/or video)received via the direct mode channel, and correspondingly updating theidentity of the last radio to transmit. At step 420, the radio may alsoprovide some audio or visual indication to its user, perhaps via adisplay such as screen 305 or speaker such as speaker 320, that the newcall requested at step 404 cannot be completed (e.g., was denied). Onthe other hand, if it is determined that the call is not of interest tothe initiating direct mode radio at step 418, processing proceeds tostep 422, where the initiating direct mode radio refrains from joiningthe new call, but may still provide some audio or visual indication toits user, perhaps via the display or the speaker, that the new callrequested at step 404 cannot be completed, and may still andcorrespondingly update the identity of the last radio to transmit.

Returning to step 416, if it is determined that no other call grantshave been received on the direct mode channel, the initiating directmode radio may assume that there was some problem in transmitting thenew call request to the arbitrating radio, some problem in thearbitrating radio processing the request, or some problem in receivingthe call grant at the initiating radio, among other possibilities.Processing thus proceeds to step 424, where the initiating radiodetermines whether a maximum number of retries has been reached. Forexample, if the identified last direct mode radio is known at step 406,the maximum number of retries may be in the range of 2-5. On the otherhand, if the identified last direct mode radio is not known, or the lastdirect mode radio actively withdrew its arbitrator status, the maximumnumber of retries may be reduced to the range of 1-3. In any event, ifthe maximum number of retries has been reached at step 424 (e.g., theretry counter is equal to or greater than the maximum), processingproceeds to step 414, where the initiating direct mode radio initiatesthe new call on the direct mode channel despite the non-receipt of acall grant from a known or unknown arbitrating radio (and assuming thatthe channel is determined to be clear).

Returning to step 424, if it is determined that the maximum number ofretries has not been reached, processing proceeds to step 426, where theretry counter is incremented and a random delay (configurable andpreferably within the range of 0 to 720 ms) applied before proceedingback to step 408 and re-transmitting the new call request to theidentified last direct mode radio. The random delay (back-off) appliedat step 426 is intended to prevent a recurrence of interference betweentwo initiating direct mode radios that may have caused the last new callrequest transmitted at step 408 to not be fully or accurately receivedby the last transmitting direct mode radio.

FIG. 5 sets forth an example process flow 500 in which anarbitrating/last transmitting radio device helps to resolve callcollisions in a digital conventional direct mode communications system.At step 502, the arbitrating direct mode radio transmits a first call ona direct mode channel. At step 504, and after making the lasttransmission of the first call, the arbitrating direct mode radiotransitions to an arbitrator role by switching to a receive mode whiletuned to the direct mode channel and monitoring the direct mode channelfor any new direct mode call requests from other direct mode radios thattransmit with a same system (or group) identifier with which thearbitrating direct mode radio is associated. Also at step 504, and whilein the receive mode, the arbitrating direct mode radio receives a firstnew call request on the direct mode channel, addressed to thearbitrating direct mode radio or some other radio, and including asource ID identifying the requesting (e.g., initiating) direct moderadio.

At step 506, the arbitrating direct mode radio continues to monitor thedirect mode channel for a period of time (up to a configurable maximumperiod of time such as 180 ms) to determine whether any additional newcall requests have been transmitted. For example, the arbitrating directmode radio may continue monitoring the direct mode channel for 120 ms toaid in preventing a collision of call grant with a delayed call requestfrom another radio. While acting as an arbitrator at step 506, and inresponse to one of a received instruction to power-off or otherwisedisable its transceiver, or in response to an error condition indicativeof an inability to receive transmissions over the direct modecommunications channel, the arbitrating direct mode radio may broadcastan announcement indicating that the radio will no longer be available toarbitrate new call requests on the direct mode channel.

At step 508, the arbitrating direct mode radio determines whether anyother new call requests were received. If not, processing continues tostep 510, where the arbitrating direct mode radio broadcasts a callgrant granting the first new call request to the requesting direct moderadio. On the other hand, if additional other new call requests werereceived at step 508, processing proceeds to step 512, where thearbitrating direct mode radio arbitrates between the two or more newcall requests received. The arbitrating direct mode radio may use anynumber of known techniques for determining which call to grant,including considering an order in which the requests were received(giving higher priority to earlier arriving requests), a priorityassociated with a particular radio (perhaps pre-stored at thearbitrating direct mode radio, based on a radio ID included in therequest), a priority associated with a talkgroup to which the new callrequest is directed (again, perhaps pre-stored at the arbitrating directmode radio, based on a radio ID included in the request), based on thetype of call requested (emergency vs. non-emergency, group vs. private,etc.), or some other system or radio attribute. Assuming that the firstnew call request is determined at step 512 as entitled to priority overthe other new call requests received at step 506, processing proceeds tostep 510, where the arbitrating direct mode radio broadcasts a callgrant granting the first new call request to the requesting direct moderadio, the call grant identifying a source ID of the requesting directmode radio.

At step 514, the arbitrating direct mode radio determines whether it isinterested in the granted first new call (perhaps in a manner similar tostep 418 of FIG. 4), and if so, receives and plays back the contentreceived from the requesting first direct mode radio over the directmode channel, and correspondingly updates the identity of the last radioto transmit, at step 516. If, on the other hand, the arbitrating directmode radio determines that it is not interested in the granted first newcall, it refrains from receiving, decoding, and/or playing back thecontent transmitted by the first direct mode radio, but may stillcorrespondingly update the identity of the last radio to transmit.

In the foregoing specification, specific embodiments have beendescribed. However, one of ordinary skill in the art appreciates thatvarious modifications and changes can be made without departing from thescope of the invention as set forth in the claims below. Accordingly,the specification and figures are to be regarded in an illustrativerather than a restrictive sense, and all such modifications are intendedto be included within the scope of present teachings.

The benefits, advantages, solutions to problems, and any element(s) thatmay cause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeatures or elements of any or all the claims. The invention is definedsolely by the appended claims including any amendments made during thependency of this application and all equivalents of those claims asissued.

Moreover in this document, relational terms such as first and second,top and bottom, and the like may be used solely to distinguish oneentity or action from another entity or action without necessarilyrequiring or implying any actual such relationship or order between suchentities or actions. The terms “comprises,” “comprising,” “has”,“having,” “includes”, “including,” “contains”, “containing” or any othervariation thereof, are intended to cover a non-exclusive inclusion, suchthat a process, method, article, or apparatus that comprises, has,includes, contains a list of elements does not include only thoseelements but may include other elements not expressly listed or inherentto such process, method, article, or apparatus. An element proceeded by“comprises . . . a”, “has . . . a”, “includes . . . a”, “contains . . .a” does not, without more constraints, preclude the existence ofadditional identical elements in the process, method, article, orapparatus that comprises, has, includes, contains the element. The terms“a” and “an” are defined as one or more unless explicitly statedotherwise herein. The terms “substantially”, “essentially”,“approximately”, “about” or any other version thereof, are defined asbeing close to as understood by one of ordinary skill in the art, and inone non-limiting embodiment the term is defined to be within 10%, inanother embodiment within 5%, in another embodiment within 1% and inanother embodiment within 0.5%. The term “coupled” as used herein isdefined as connected, although not necessarily directly and notnecessarily mechanically. A device or structure that is “configured” ina certain way is configured in at least that way, but may also beconfigured in ways that are not listed.

It will be appreciated that some embodiments may be comprised of one ormore generic or specialized processors (or “processing devices”) such asmicroprocessors, digital signal processors, customized processors andfield programmable gate arrays (FPGAs) and unique stored programinstructions (including both software and firmware) that control the oneor more processors to implement, in conjunction with certainnon-processor circuits, some, most, or all of the functions of themethod and/or apparatus described herein. Alternatively, some or allfunctions could be implemented by a state machine that has no storedprogram instructions, or in one or more application specific integratedcircuits (ASICs), in which each function or some combinations of certainof the functions are implemented as custom logic. Of course, acombination of the two approaches could be used.

Moreover, an embodiment can be implemented as a computer-readablestorage medium having computer readable code stored thereon forprogramming a computer (e.g., comprising a processor) to perform amethod as described and claimed herein. Examples of suchcomputer-readable storage mediums include, but are not limited to, ahard disk, a CD-ROM, an optical storage device, a magnetic storagedevice, a ROM (Read Only Memory), a PROM (Programmable Read OnlyMemory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM(Electrically Erasable Programmable Read Only Memory) and a Flashmemory. Further, it is expected that one of ordinary skill,notwithstanding possibly significant effort and many design choicesmotivated by, for example, available time, current technology, andeconomic considerations, when guided by the concepts and principlesdisclosed herein will be readily capable of generating such softwareinstructions and programs and ICs with minimal experimentation.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in various embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments require morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive subject matter lies in less than allfeatures of a single disclosed embodiment. Thus the following claims arehereby incorporated into the Detailed Description, with each claimstanding on its own as a separately claimed subject matter.

We claim:
 1. A method for resolving call collisions in a direct modecommunication system among a plurality of direct mode radios, the methodcomprising, at an initiating direct mode radio: monitoring a direct modecommunication channel for transmissions from other direct mode radios inthe plurality of direct mode radios; responsive to detecting a requestto initiate a new direct mode call on the direct mode communicationchannel: identifying, via the monitoring, a last one of the other directmode radios to transmit on the direct mode communication channel;transmitting a new call request for receipt by the last one of the otherdirect mode radios to transmit on the direct mode communication channel;monitoring the direct mode communication channel for a response from thelast one of the other direct mode radios to transmit on the direct modecommunication channel; if a call grant is received from the last one ofthe other direct mode radios to transmit on the direct modecommunication channel granting the new call request, initiating the newdirect mode call on the direct mode communication channel; and if thecall grant is not received from the last one of the other direct moderadios to transmit on the direct mode communication channel granting thenew call request, at least temporarily refraining from initiating thenew direct mode call on the direct mode communication channel.
 2. Themethod of claim 1, further comprising: if the call grant is not receivedfrom the last one of the other direct mode radios to transmit on thedirect mode communication channel granting the new call request, but asecond call grant is received granting a second call request fromanother of the other direct mode radios to transmit on the direct modecommunication channel, determining if the second call request is ofinterest, and if so, unmuting and playing back content transmitted inthe second call by the another of the other direct mode radios.
 3. Themethod of claim 1, further comprising: if the call grant is not receivedfrom the last one of the other direct mode radios to transmit on thedirect mode communication channel granting the new call request, and nocall grant of any other of the other direct mode radios to transmit onthe direct mode communication channel is detected, waiting for a randomperiod of time and then re-transmitting the call request directly to thelast one of the other direct mode radios to transmit on the direct odecommunication channel.
 4. The method of claim 3, further comprising:after re-transmitting the call request directly to the last one of theother direct mode radios to transmit on the direct mode communicationchannel a threshold number of times without receiving a call grantgranting the new call request and without detecting any other callgrant, initiating the new direct mode call on the direct modecommunication channel despite the non-receipt of the call grant from thelast one of the other direct mode radios.
 5. The method of claim 1,wherein: monitoring the direct mode communication channel fortransmissions from other direct mode radios in the plurality of directmode radios comprises monitoring the direct mode communication channelfor transmissions including a particular system identifier with whichthe initiating direct mode radio is also associated with and includes inits transmissions.
 6. The method of claim 5, wherein: identifying thelast one of the other direct mode radios to transmit on the direct modecommunication channel comprises identifying a last one of the otherdirect mode radios to transmit on the direct mode communication channela transmission including the particular system identifier.
 7. The methodof claim 1, wherein: monitoring the direct mode communication channelfor transmissions from other direct mode radios in the plurality ofdirect mode radios comprises, retrieving, via one of a link controlheader frame and embedded link control frame, an identity of a sourceradio transmitting on the direct mode communication channel and storingthe identity; identifying the last one of the other direct mode radiosto transmit on the direct mode communication channel comprisesretrieving the stored identity of the last source radio to transmit onthe direct mode communication channel; and transmitting the call requestfor receipt by the last one of the other direct mode radios to transmiton the direct mode communication channel comprises populating a targetdevice identifier field of a voice header or preamble control signalingblock (CSBK), forming the call request, with the retrieved storedidentity of the last source radio to transmit on the direct modecommunication channel.
 8. The method of claim 1, further comprising:prior to detecting the request to initiate the new direct mode call, andduring the monitoring, receiving an announcement from the last one ofthe last one of the other direct mode radios to transmit on the directmode communication channel indicating that it will no longer beavailable to arbitrate new call requests; and re-transmitting the callrequest a reduced number of times compared to a number of times if theannouncement had not been received, and responsive to not receiving thecall grant from the last one of the other direct mode radios to transmiton the direct mode communication channel granting the new call request,initiating the new direct mode call on the direct mode communicationchannel despite the non-receipt of the call grant from the last one ofthe other direct mode radios.
 9. The method of claim 1, wherein theplurality of direct mode radios communicate in accordance with aEuropean Telecommunications Standard Institute-Digital Mobile Radio(ETSI-DMR) direct mode protocol.
 10. A method for resolving callcollisions in a direct mode communication system among a plurality ofdirect mode radios, the method comprising, at a first direct mode radioout of the plurality of direct mode radios: transmitting, on a directmode communication channel, one or more transmissions during a firstdirect mode call to the other direct mode radios in the plurality ofdirect mode radios, the transmissions including an identifieridentifying the first direct mode radio; after making the lasttransmission of the first direct mode call, transitioning to anarbitrating mode for the direct mode communication channel andmonitoring the direct mode communication channel for any new direct modecall requests from other direct mode radios in the plurality of directmode radios.
 11. The method of claim 10, further comprising: receiving,from a second direct mode radio out of the plurality of direct moderadios, a first new direct mode call request; monitoring the direct modecommunication channel for any additional new direct mode call requestsfrom other direct mode radios in the plurality of direct mode radios;and responsive to determining that one of (i) no other new direct modecall requests have been received and (ii) other new direct mode callrequests have been received but the first new direct mode call requestis entitled to priority over the other new call requests: broadcasting acall grant granting the first new direct mode call request of the seconddirect mode radio, the call grant including an identifier identifyingthe second direct mode radio; and receiving and playing back contentreceived from the second direct mode radio broadcast over the directmode communication channel during the first new direct mode call grantedby the acknowledgment.
 12. The method of claim 11, further comprisingdetermining that no other direct mode call requests are received. 13.The method of claim 11, further comprising receiving other new directmode call requests, determining that other new direct mode call requestshave been received, and determining that the first new direct mode callrequest is entitled to priority over the other new call requests. 14.The method of claim 11, further comprising: responsive to receiving andplaying back content received from the second direct mode radio,identifying the second direct mode radio as the last one of the otherdirect mode radios to transmit on the direct mode communication channeland storing an identity of the second direct mode radio for use inarbitrating future new call requests.
 15. The method of claim 11,wherein: the direct mode call request comprises a voice header orpreamble control signaling block (CSBK) including a target deviceidentifier field populated with a device identifier associated with thefirst direct mode radio.
 16. The method of claim 10, wherein: monitoringthe direct mode communication channel for transmissions from otherdirect mode radios in the plurality of direct mode radios comprisesmonitoring the direct mode communication channel for transmissionsincluding a particular system identifier with which the first directmode radio is also associated with and includes in its transmissions.17. The method of claim 10, further comprising: responsive to one of areceived instruction to power-off or otherwise disable its transceiveror an error condition indicative of an inability to receivetransmissions over the direct mode communications channel, broadcastingan announcement indicating that the first direct mode radio will nolonger be available to arbitrate new call requests on the direct modechannel.
 18. The method of claim 10, wherein the plurality of directmode radios communicate in accordance with a European TelecommunicationsStandard Institute-Digital Mobile Radio (ETSI-DMR) direct mode protocol.19. A direct mode radio comprising: a memory; a transceiver comprising atransmitter and a receiver; and a processor configured to: monitor, viathe receiver, a direct mode communication channel for transmissions fromother direct mode radios in the plurality of direct mode radios;responsive to detecting a request to initiate a new direct mode call onthe direct mode communication channel via the receiver: identify, viathe monitoring, a last one of the other direct mode radios to transmiton the direct mode communication channel; transmit, via the transmitter,a new call request for receipt by the last one of the other direct moderadios to transmit on the direct mode communication channel; monitor,via the receiver, the direct mode communication channel for a responsefrom the last one of the other direct mode radios to transmit on thedirect mode communication channel; if a call grant is received from thelast one of the other direct mode radios to transmit on the direct modecommunication channel granting the new call request, initiate, via thetransmitter, the new direct mode call on the direct mode communicationchannel; and if the call grant is not received from the last one of theother direct mode radios to transmit on the direct mode communicationchannel granting the new call request, at least temporarily refrain frominitiating the new direct mode call on the direct mode communicationchannel.